1. Field of the Invention
This invention relates to a method of making a characteristic line, and more particularly to a method of making a characteristic line representing the relation between a signal value of a photodetector element which outputs a signal of a value depending upon the amount of light which it receives and the amount of light which the photodetector element receives.
2. Description of the Related Art
When a photodetector element which outputs a signal of a value depending upon the amount of light which it receives is used, there has been known a system where a characteristic line representing the relation between a signal value of the photodetector element and the amount of light which it receives is obtained and the amount of light which the photodetector element receives is obtained from the signal value output from the photodetector element on the basis of the characteristic line. (See Japanese Unexamined Patent Publication No. 2001-137175)
As a procedure for obtaining the characteristic line, there has been known, for instance, that where a value of a signal output from a photodetector element with the photodetector element receiving no light and at the same time while a value of a signal output from the photodetector element with the photodetector element receiving an amount of light near to a maximum which the photodetector element can receive is obtained, and the characteristic line is obtained on the basis of the relation between the amount of light which the photodetector element receives and the output of the photodetector element of each of the pairs of amount of light and the output. That is, in the procedure, the signal value output from the photodetector element is converted to a light amount by the use of the characteristic line.
When such a characteristic line is applied to a line sensor in which a number of photodetector elements are arranged in one direction, there has been known a method in which the characteristic line is obtained element to element and the signal values output from the respective photodetector elements are weighted so that they output the same signal values when they receive the same amount of light.
However, in the photodetector element, the relation between the amount of light which the photodetector element receives and the output of the photodetector element is slightly nonlinear on the side where the amount of light which the photodetector element receives is small. Accordingly, there has been a demand to obtain a more accurate amount of light even on the side of small amount of light.
To the contrast, a method where relations of a plurality of pairs of amount of light and the signal value are obtained and stored in a look-up table on the side of small amount of light and the signal value exhibiting the amount of light which the photodetector element actually receives is obtained without use of the characteristic line by interpolation between points on a two-dimensional coordinate system representing a relation between the amount of light which the photodetector element receives and the signal value (one axis representing the amount of light and the other axis representing the signal value) on the basis of the look-up table is conceivable.
However, the method using the look-up table is disadvantageous in that the system is complicated since the look-up table must be referred to every time the amount of light which the photodetector element receives (will be referred to as “the photodetector receiving light amount”, hereinbelow) is obtained. Further, attempt to apply the method to a line sensor will further complicate the system and the problem will become more serious since the look-up tables must be prepared for all the photodetectors forming the line sensor.
Accordingly, it is required to make more accurate the relation between the photodetector receiving light amount and the signal value on the side of small amount of light while using the characteristic line.